Method of making carbon nanotubes doped with iron, nitrogen and sulphur

ABSTRACT

A method of making carbon nanotubes doped with iron, nitrogen and sulphur for an oxygen reduction reaction catalyst includes the steps of mixing an iron containing oxidising agent with a sulphur-containing dye to form a fibrous fluctuate of reactive templates and using these for in-situ polymerisation of an azo compound to form polymer-dye nanotubes, adding an alkali to precipitate magnetite, and subjecting the nanotubes to pyrolysis, acid leaching, and heat treatment.

FIELD OF INVENTION

The invention relates to a method of making carbon nanotubes doped withiron, nitrogen and sulphur.

BACKGROUND

A Polymer Electrolyte Membrane (PEM) fuel cell typically compriseselectrodes, an electrolyte, a catalyst, and gas diffusion layers. Amixture of catalyst, carbon, and electrode is coated onto the solidelectrolyte and carbon is hot pressed on either side to protect theinside of the cell and also act as electrodes. The cell reactions occurat the triple phase boundary (TPB) where the electrolyte, catalyst, andreactants mix.

The membrane conducts hydrogen ions but not electrons and it must alsonot allow gas from one side of the cell to pass to the other. Splittingof the hydrogen molecule is relatively easy by using a platinumcatalyst.

Oxygen reduction reaction (ORR) catalysts play an essential role inlarge-scale implementation of PEM fuel cells and those on the markettend to use noble metals, such as platinum, iridium and ruthenium, dueto their low overpotential and high current density. However, suchcatalysts suffer from problems like high cost, scarcity, aggregation inalkaline electrolytes, susceptibility to methanol, and carbon monoxide(CO) poisoning

Transition metal-based and heteroatom-doped carbon materials areregarded as promising replacements for commercial catalysts in oxygenreduction reactions for PEM fuel cells and metal-air batteries.

However, in most cases researchers mainly focus on introducing foreignspecies on the surface or in the void space of carbon nanostructures,potentially leading to loose attachment and aggregation of the dopants,thus the synergetic effect between the dopant and carbon structure iscompromised.

An aim of the invention therefore is to provide a material for an ORRcatalyst which overcomes the above issues.

SUMMARY OF INVENTION

In an aspect of the invention, there is provided a method of makingcarbon nanotubes doped with iron, nitrogen and sulphur, comprising thesteps of:

-   -   mixing an iron containing oxidising agent with a        sulphur-containing dye to form a fibrous fluctuate of reactive        templates;    -   adding an azo compound to the reactive templates for in-situ        polymerisation to form polymer-dye nanotubes;    -   adding an alkali to precipitate magnetite embedded in the walls        of the nanotubes;    -   subjecting the nanotubes to pyrolysis for initiating        carbonisation of the azo polymer and for decomposing the        sulphur-containing dye;    -   subjecting the nanotubes to acid leaching for removing        superfluous materials; and    -   subjecting the nanotubes to heat treatment for further        decomposing the sulphur-containing dye, activating iron species        and further carbonising the azo polymer.

Advantageously the method is simple and scalable, and the resultingcatalyst showed excellent ORR performance comparable to state-of-the-artplatinum/carbon catalysts in alkaline media due to the synergisticeffect between the iron and metalloid elements which is reinforced bythe intimate contact between the iron-containing nanoparticles and thecarbon walls doped with nitrogen and sulphur. It is also a promisingcandidate for the electrodes of supercapacitors, metal-air batteries andgas adsorbents, etc.

In one embodiment the reactive templates are self-degrade nanowiretemplates.

In one embodiment the iron containing oxidising agent is iron(iii)chloride (FeCl₃) or iron(iii) nitrate (Fe(NO₃)₃).

In one embodiment the iron containing oxidising agent andsulphur-containing dye are mixed in a ratio of around 3:1.

In one embodiment the sulphur-containing dye is methyl orange.

In one embodiment the azo compound is a pyrrole, aniline, carbazole,indole or the like. Typically the azo polymer is a polypyrrole,polyaniline, polycarbazole, polyindole, or the like.

In one embodiment, after the azo compound is added to the reactivetemplates, the mixture is stirred at room temperature for about 24hours. The azo compound polymerises around the fibrous template throughoxidation by the iron species in the template, driving the iron speciesto diffuse out from the inner fibrous core towards the polymer shells,leading to hollow nanotubes.

In one embodiment the nanotubes have a diameter ranging from around 20to about 200 nanometres.

In one embodiment the alkali is sodium hydroxide or potassium hydroxide.Typically the alkali has a concentration of 0.5 M. Typically thenanotubes are in the form of a black fluffy powder.

In one embodiment the pyrolysis step comprises subjection totemperatures of around 400-600° C. for about two hours. This triggersthe preliminary thermal carbonisation of the nanotubes and betteranchors iron oxide particles in the nanotube wall. In one embodimentthis step takes place in an atmosphere of inert gas, typically argon.

In one embodiment the carbon in the nanotubes is substantially derivedfrom the carbonisation processes.

In one embodiment the azo polymer serves as the source of the nitrogendopant.

In one embodiment the sulphur-containing dye serves as the source of thesulphur dopant.

In one embodiment the iron-containing oxidising agent serves as thesource of the iron dopant.

In one embodiment the nitrogen and sulphur dopants are homogeneouslydistributed in the carbon nanotube.

In one embodiment the acid leaching step removes superfluous less-activeoxide materials.

In one embodiment the heat treatment step comprises subjection totemperatures of around 700-900° C. for about 5 hours. In one embodimentthis step takes place in an atmosphere of inert gas, typically argon.

In a further aspect of the invention there is provided a composition ofcarbon nanotubes doped with iron, nitrogen and sulphur made according tothe method described herein.

In one embodiment the iron is in the form of magnetite crystals. In oneembodiment the magnetite crystals are embedded in the wall of thenanotubes.

In a yet further aspect of the invention the composition is used as afunctional material in an electrode, filter, absorber, catalyst, sensor,and/or the like. In one embodiment the composition is used as an oxygenreduction reaction catalyst.

BRIEF DESCRIPTION OF DRAWINGS

It will be convenient to further describe the present invention withrespect to the accompanying drawings that illustrate possiblearrangements of the invention. Other arrangements of the invention arepossible, and consequently the particularity of the accompanyingdrawings is not to be understood as superseding the generality of thepreceding description of the invention.

FIG. 1 illustrates the steps for making doped carbon nanotubes accordingto an embodiment of the invention.

FIG. 2 illustrates (a) XRD spectrum of Fe-NSCNT-500 and Fe-NSCNT-800,(b) SEM image, (c) low magnification and (d) high magnification TEMimages of Fe-NSCNT-800.

FIG. 3 illustrates (a) Survey scan of XPS spectrum of Fe-NSCNT andNSCNT. (b) N1s, (c) S2p, (d) Fe 2p, (e) N₂ adsorption isotherms ofFe-NSCNT-800 and NSCNT-800, (f) BJH method size distribution.

FIG. 4 illustrates (a) CV curves of Fe-NSCNT-800 and commercial 20% Pt/Ccatalyst in Ar-saturated (dotted line) and O₂-saturated (solid line)solution, respectively, (b) Linear sweep voltammetry (LSV) curves ofFe-NSCNT-800, NSCNT-800 and 20% Pt/C catalysts in O₂-saturated 0.1 M KOHsolution at a sweep rate of 10 mV s⁻¹ and the electrode rotation speedof 1600 rpm, (c) LSV curves for Fe-NSCNT-800 at various rotation rates,inset show corresponding Koutecky-Levich plots at different potentials,(d) Rotating ring-disk electrode voltammograms recorded in O₂-saturated0.1 M KOH at 1,600 rpm. Disk current (I_(d)) (solid line) is showed onthe lower half and the ring current (I_(r)) (dotted line) is showed onthe upper half of the graph.

FIG. 5 illustrates (a) Current-time (I-t) curves for Fe-NSCNT-800 andthe 20% Pt/C catalyst in O₂-saturated KOH (0.1 M) solution with theaddition of methanol (final concentration of 3 M). (b) I-t curves forFe-NSCNT-800 and the 20% Pt/C catalyst in O₂-saturated KOH (0.1 M)solution at −0.3 V versus Hg/HgO. (c) LSV curves for Fe-NSCNT-800 atvarious rotation rates in 0.5 M H₂SO₄.

FIG. 6 illustrates performance of a primary Zn-air battery (a)Polarization and power density curves of primary Zn-air batteries usingFe-NSCNT and 20% Pt/C (mass loading of 1 mg cm⁻²) and 6 M KOHelectrolyte (scan rate, 5 mVs⁻¹). (b) Specific capacities of the Zn-airbatteries using Fe-NSCNT as ORR catalyst are normalized to the mass ofthe consumed Zn. (c) Discharge curves of the primary Zn-air batteriesusing Fe-NSCNT and 20% Pt/C as the ORR catalyst and the KOH electrolyteat 10 and 20 mA cm⁻². (d) Photograph of the two Zn-air batteries (usingthe Fe-NSCNT-800 cathode) connected in series lighting an LED bulb.

FIG. 7 illustrates Raman spectra of (a) Fe-NSCNT-800 and NSCNT-800 and(b) Fe-NSCNT-700, Fe-NSCNT-800 and Fe-NSCNT-900.

FIG. 8 illustrates a comparison of electron transfer number anddiffusion limiting current density among (a) NSCNT-800, Fe-NSCNT-800 and20% Pt/C, and (b) the Fe-NSCNT catalysts fabricated with differenttemperature used at the last annealing step.

FIG. 9 illustrates LSV curves of Fe-NSCNT-800, NSCNT-800 and 20% Pt/C inthe O₂-saturated H₂SO₄ (0.05 M) solution measured at a sweep rate of 10mV s⁻¹ and an electrode rotation speed of 1600 rpm.

FIG. 10 illustrates OCV of Zn-air batteries constructed usingFe-NSCNT-800 and 20% Pt/C

DETAILED DESCRIPTION

With regard to FIG. 1, the overall method of making carbon nanotubesdoped with iron, nitrogen and sulphur is illustrated.

In the first step, 5 mmol methyl orange 2 was dissolved in 30 mldeionized water, and then mixed 6 with an iron containing oxidisingagent such as 1.5 mmol iron(iii) chloride (FeCl₃) to form a fibrousfluctuate comprising reactive templates 8.

An azo compound such as 105 μl pyrrole 12 is then added 10 which reacts14 in an in-situ polymerisation, coating the templates with polypyrrole16. The iron(iii) (Fe³⁺) ions 20 from the inner fibre migrate out tooxidize the pyrrole, leaving iron(ii) (Fe²⁺) ions 18 inside, resultingin the formation polypyrrole-methyl orange nanotubes 22.

An alkali such as 50 ml 0.5 M sodium hydroxide (NaOH) is then added 24,which causes magnetite (Fe₃O₄) 26 to precipitate in the walls of thenanotubes. The precipitate was filtered, washed with deionizedwater/ethanol, and dried in vacuum at 60° C. for around 24 hours.

In the last step 28 the nanotubes are ground into fluffy powder,transferred into a lidded ceramic boat, and subjected to pyrolysis ataround 500° C. for initiating carbonisation of the polypyrrole. Thecooled sample in the form of black powder was then immersed in 6 M HClsolution for around 12 hours (acid leaching) to remove the superfluousiron materials, and then after washing and drying the powder wassubjected to heat treatment in argon for around 5 hours at around700-900° C. for decomposing the methyl orange, activating iron speciesand further carbonising the polypyrrole.

The resulting iron nitrogen sulphur tri-doped carbon nanotubes 30(Fe-NSCNT) 30 offer the benefits of carbon nanotubes such as goodelectrical conductivity, high surface area and excellent mechanicalproperty, with the doped heteroatoms enabling modified electronic andchemical characteristics, optimizing the catalytic performance. TheFe₃O₄ crystals are embedded in the walls of the carbon nanotubes therebysubstantially reinforcing the synergetic effect with the carbon andincreasing ORR performance.

The carbon nanotubes include nanopores 32 which increases surface areaand the number of catalytic active sites.

Materials Characterizations

The morphology of the as-prepared samples were investigated usingscanning electron microscopy (SEM, Philips XL-30 FESEM), andtransmission electron microscopy (TEM, JEOL TEM 2100F FEG operated withan accelerating voltage of 200 kV). Raman spectrum was conducted with aRenishaw-200 visual Raman microscope (633 nm in wavelength). The X-raydiffraction (XRD) patterns were collected using an X-ray diffractometer(Rigaku SmartLab) using Cu Kα radiation. Brunauer-Emmett-Teller (BET)surface area and total pore volume were tested on a Micromeritics,ASAP2020 gas sorption analyzer at 77K. X-ray photoelectron spectroscopy(XPS) measurements were carried out on a VG ESCALAB 220i-XL surfaceanalysis system.

Electrochemical Measurements

4 mg catalyst was dispersed in 400 μL of 0.5 wt % Nafion solutionultrasonically to form homogeneous slurry. The slurry (8 μL) was thentransferred onto a glassy carbon electrode with a catalyst loading of409 μg cm⁻². The catalyst-coated glassy carbon electrode (GCE, 0.19625cm²), Hg/HgO (KOH, 0.1 M), and Pt wire were used as the working,reference and counter electrode, respectively. Electrochemicalmeasurements were conducted on a Biologic VMP3 electrochemical stationwith a three-electrode cell system at room temperature. For comparison,Pt/C powder was purchased from Alfa Aesar and tested.

All the electrochemical experiments were performed in O₂- orAr-saturated 0.1 M KOH (0.5 M H₂SO₄) electrolyte. Cyclic voltammograms(CVs) were performed between −0.8 and +0.2 V versus Hg/HgO in 0.1 M KOH(or between −0.4 and 0.7 V versus SCE in 0.5 M H₂SO₄), at a scan rate of10 mV s⁻¹. The rotating disk electrode (RDE) was investigated atdifferent rotating speed from 400 to 2500 rpm, and the rotatingring-disc electrode (RRDE) at 1600 rpm. The electron transfer number (n)was determined from the following equations (eq. 1):

$\begin{matrix}{n = {4 \times \frac{I_{d}}{I_{d} + {I_{r}/N}}}} & (1)\end{matrix}$where N=0.37 is the collection efficiency, I_(d) and I_(r) are the diskand ring current, respectively.

The current density (J_(K)) of the samples was analyzed by RDE andcalculated on the basis of the Koutecky-Levich equations (eqs. 2-4):

$\begin{matrix}{\frac{1}{J} = {{\frac{1}{J_{L}} + \frac{1}{J_{K}}} = {\frac{1}{B\;\omega^{1/2}} + \frac{1}{J_{K}}}}} & (2) \\{B = {0.62\; n\; F\;{C_{0}\left( D_{0} \right)}^{2/3}c^{{- 1}/6}}} & (3) \\{J_{K} = {n\; F\; k\; C_{0}}} & (4)\end{matrix}$where J, J_(K), J_(L) are the measured current density, kinetic anddiffusion limiting current densities, respectively, co is the angularvelocity of the disk, B is the reciprocal of the slope, which can bedetermined from the slope of K-L plot using Levich equation, n is theelectron transferring number, F is the Faraday constant (96485 C mol⁻¹),C₀ is the bulk concentration of O₂, D₀ is the diffusion coefficient ofO₂, ν is the viscosity of the electrolyte, and k is the electrontransfer rate constant.Zinc-Air Battery Tests

The measurements of the zinc-air batteries were performed usinghome-built electrochemical cells. The catalyst-coated carbon paper wasused as the air cathode (catalyst mass loading of 1 mg cm⁻²), while a Znfoil (2.5 cm*2 cm*0.3 mm) was employed as the anode. The zinc-airbattery was fabricated by pairing the cathode and anode in 28 ml of 6 MKOH. Linear sweep voltammetry (LSV) was conducted within a range of 1.6V-0.4 V at 5 mV s⁻¹ using a potentiostat (CHI 660E). A battery cycler(Neware) was used to measure the galvanostatic discharge curves.

FIG. 2 illustrates that the acid treatment removes most of thesuperficial Fe₃O₄ precipitate and leaves behind only the highlycatalytically active Fe₃O₄ nanoparticles that are enchased in the wallsof N/S-doped carbon nanotubes.

In more detail, the XRD patterns (FIG. 2a ) of Fe-NSCNT-precursor showeddiffraction peaks at 30.48°, 35.52°, 43.32°, 53.36°, 57.01°, 62.73° and74.35° which are assigned to the (220), (311), (400), (422), (511) and(440) of Fe₃O₄ (ICDD No. 27-1402), respectively. However, no diffractionfeatures of Fe₃O₄ were observed for Fe-NSCNT-800 (FIG. 2b ), confirmingthe removal of most iron species upon acid leaching. Notably, two broadpeaks located at 24.8 and 42.8° were observed in Fe-NSCNT-800 andNSCNT-800, corresponding to the (002) planes of graphitic carbon and the(100) planes of a hexagonal carbon material (JCPDS No. 75-1621),respectively, indicating the presence of graphitic carbon inFe-NSCNT-800 and NSCNT-800. Furthermore, Raman spectrum (FIG. 7) showsthat the Fe-NSCNT has higher degree of graphitic ordering than N, S-CNT(I_(D)/I_(G)=0.9 and lower 2D peak) and thus higher conductivity,beneficial for the oxygen reduction process.

SEM and TEM studies (FIG. 2b-d ) clearly shows the nanotubularmorphology of Fe-NSCNT-800. Dissociative particles were spotted in thenanotube walls of the Fe-NSCNT-precursor, rather than Fe-NSCNT-800 (FIG.2c ), indicating that the acid treatment washes away almost all theFe₃O₄ particles that were loosely attached onto the NSCNT wall, which isin good agreement with the XRD results discussed above. Further TEMobservation reveals that the Fe-containing nanoparticles were embeddedin the tube walls for Fe-NSCNT-800 (FIG. 2c ). The lattice spacing ofthe particles was measured 0.485 nm from the high resolution TEM images(FIG. 2d ), corresponding to the (111) planes of Fe₃O₄. As will be shownin a later part of this article, the great synergistic effect betweenthe Fe₃O₄ and the N, S-doped carbon tubes attributes to the superior ORRperformance of Fe-NSCNT-800. For comparison, no nanoparticles were foundin the wall of NSCNT-800.

FIG. 3 confirms that the successful incorporation of the Fe species inN/S-doped carbon nanotubes (Fe-NSCNT-800), and the high surface area andhierarchical mesoporous morphology of Fe-NSCNT-800.

In more detail, X-ray photoelectron spectroscopy (XPS) measurements(FIG. 3a ) revealed the presence of C, N, S, O and Fe in Fe-NSCNT-800,whereas no Fe was detected in NSCNT-800. The high-resolution Fe 2pspectra (FIG. 3b ) further confirmed that no Fe 2p signal was detectedfor NSCNT-800, while Fe-NSCNT-800 exhibited clear Fe 2p peaks that canbe deconvoluted into five peaks at 710.7, 713.6, 718.7, 722.9, and 725.2eV. The peaks at 710.7 and 713.6 eV can be assigned to 2p3/2 of Fe (III)and Fe (II) ions, respectively. The peak at 722.9 eV can be assigned to2p1/2 of Fe(II) ion, and the peak at 725.2 eV to 2p1/2 of Fe(II) andFe(III) ions. The peak at 718.7 eV was a satellite peak for the abovefour peaks, indicating the co-existence of Fe(II) and Fe(III) at a moleratio (II:III) of ˜1.8. It can be concluded from the XPS results that Fein Fe-NSCNT-800 was mainly present in the form of Fe₃O₄.

Fittings of the N is spectra of Fe-NSCNT-800 and NSCNT-800 (FIG. 3c )both showed 4 peaks assigned to pyridinic N (˜398.7 eV), pyrrolic N(˜400.4 eV), graphitic N (˜401.2 eV), and oxidized N (403.8 eV and 402.4eV). The peaks at 398.7 eV for Fe-NSCNT-800 possibly containcontribution from the bonding between N and Fe (N—Fe), considering thebinding energy of pyridinic N is almost same as N—Fe. It should bepointed out that all of these observed N species, except the uncertaincontribution of the oxidized N, have been reported to play a crucialrole in the ORR process. The high-resolution S 2p spectrum (FIG. 3d )detected S 2p1/2 and 2p3/2 doublet at 164.9 eV and 163.7 eV spin-orbitlevels with an energy separation of 1.2 eV and an intensity ratio of1:2, due to the formation of C═S and C—S bonds, respectively, confirmingthe S doping of the carbon nanotubes. The peaks at 166-172 eVcorresponding to —C—SO_(X)—C— possibly originated from the MO species.These types of S dopants are reported to improve the ORR activity.

The N₂-adsorption/desorption isotherm (FIG. 3e ) of Fe-NSCNT-800displayed type-IV characteristics with sharp uptakes at low relativepressure (<0.05) and H3-type hysteresis loops (uptakes for relativepressure ranging from 0.45 to 1.0), indicating its mesoporous structure.The pore-size distribution of Fe-NSCNT-800 (mainly centered at 7-8.5 nm,10-13 nm and 19-29 nm, FIG. 3f ) indicated its hierarchical mesoporousstructure of, while much less mesopores, particularly for pores below 20nm, were detected in NSCNT-800 (FIG. 3f ). Moreover, the specificsurface area of Fe-NSCNT-800 (247.21 m² g⁻¹) is significantly largerthan that of NSCNT-800 (185.65 m²g⁻¹), due to the differentpost-treatment of Fe-NSCNT-800 which created more pores ranging in 7-13nm. The increased surface area and hierarchical pore structure ofFe-NSCNT-800 are favorable for producing more active sites and rapidmass transfer for ORR.

FIG. 4 reveals that Fe-NSCNT-800 has comparable or enhanced ORRperformances than both commercial 20% Pt/C and the control samples thatare without Fe-doping (NS CNT-800).

In more detail, The ORR catalytic activity of Fe-NSCNT-800 and thecommercial state-of-art 20% Pt/C catalyst were first investigated usingcyclic voltammetry (CV) in O₂- or Ar-saturated alkaline solution (0.1 MKOH) (FIG. 4a ). Featureless slopes were observed for Fe-NSCNT-800 inthe Ar-saturated electrolyte. In sharp contrast, an obvious cathodicpeak at −0.09 V (vs. Hg/HgO) appeared for Fe-NSCNT-800 in theO₂-saturated solution, which is much more prominent than the Pt/Ccatalyst, implying a superior ORR activity of Fe-NSCNT-800. The ORRperformances of Fe-NSCNT-800 were further tested by both rotating diskelectrodes (RDE) and rotation ring disk electrodes (RRDE). As shown inFIG. 4b , the Fe-NSCNT-800 catalyst exhibited extraordinary ORRperformance with a half-wave potential of −0.088 V, comparable to thatof commercial Pt/C catalyst (difference of only 25 mV). Impressively,the diffusion-limited current density of Fe-NSCNT-800 (6.1 mA cm⁻²) is1.5 times that of NSCNT-800 and significantly higher than the commercialPt black (5.8 mA cm⁻²). It was found that the ORR activity stronglydepended on the heat treatment temperature of the Fe-NSCNT samples withthe optimum temperature determined to be 800° C.: Fe-NSCNT-800 possessedthe best onset potential (E_(onset)) and diffusion-limited currentdensity, compared with Fe-NSCNT-700 and Fe-NSCNT-900. The RDEmeasurements at various rotating speeds at a scan rate of 10 mV s⁻¹ inO₂-saturated solution are carried out for Fe-NSCNT-800 (FIG. 4c ) andNSCNT-800. For both samples, the current density displays a typicalincrease with rotation rate due to the thinned diffusion layer.

Calculated using the Koutecky-Levich equations, the electron transfernumber (n) is only 3.24 for NSCNT-800 over the potential range from−0.6-−0.45 V versus Hg/HgO. In sharp contrast, n=4.03 for theFe-NSCNT-800 electrode over the same potential range (FIG. 8),indicating that it undergoes a four-electron reduction process inaqueous alkaline medium, similar to the Pt/C catalyst. The superiorcatalytic performance of Fe-NSCNT-800 is ascribed to the reinforcedsynergetic interaction between the enchased active Fe₃O₄ nanocrystalsand the highly conductive NSCNT backbone. FIG. 4d showed the RRDEpolarization curves for Fe-NSCNT-800 and NSCNT-800 loaded on a glassycarbon electrode in O₂-saturated 0.1 M KOH at 1,600 rpm, thecorresponding ring current for the oxidation of hydrogen peroxide ions(HO₂ ⁻) was measured at −0.3 V versus Hg/HgO. The ring current ofNSCNT-800 was over two times larger than Fe-NSCNT-800 in the range of−0.25 to −0.8 V versus Hg/HgO, indicating that Fe-NSCNT-800 is moreresistant to side reaction.

FIG. 5 shows that Fe-NSCNT-800 has superior methanol tolerance andlong-term stability, outperforming commercial 20% Pt/C.

In more detail, the possible crossover effect caused by small organicmolecules such as methanol is also tested by chronoamperometric (CA)(FIG. 5a ). When methanol is added into the O₂-saturated 0.1 M KOHelectrolyte (final concentration of 3 M), Fe-NSCNT-800 quickly recoversto the original current level, with no obvious decay observed. Bycontrast, for the Pt/C catalyst, addition of methanol triggers a drasticsharp surge of current density far more difficult for recovery to theinitial level, indicating its much weaker tolerance against chemicalcorrosion than Fe-NSCNT-800. The CA measurements at −0.3 V versus Hg/HgOfor Fe-NSCNT-800 (FIG. 5b ) showed no significant decay after 6 h,whereas the current of Pt/C gradually dropped by ˜19% within the sametime frame, revealing the superior catalytic stability of Fe-NSCNT-800.Furthermore, Fe-NSCNT-800 showed high ORR activity in acidic solution.The RDE measurements were carried out in 0.5 M H₂SO₄ solution over therange of −0.4-0.7 V versus SCE. The catalytic activity order of thecatalysts in acidic medium was the same as that in alkaline medium (FIG.9). The onset potential of Fe-NSCNT-800 was determined to be 0.48Vversus SCE. Moreover, the limiting current density of Fe-NSCNT-800 (5.6mA cm⁻², FIG. 5c ) at 1600 rpm was comparable to that in the alkalinesolution

FIG. 6 proves that when applied in Zn-air batteries, Fe-NSCNT-800displays extraordinary performance, very close to the commercial 20%Pt/C, indicating its great potential in replacing Pt/C for the practicalapplications of noble metal-free zinc-air batteries.

In more detail a primary Zn-air battery was assembled with Fe-NSCNT-800as the air cathode materials to further investigate its performance ofunder real battery operation conditions. A high open-circuit potential(OCP) (˜1.41 V, FIG. 10) was observed for the fabricated Zn-air battery.Furthermore, the polarization and power density curves of theFe-NSCNT-800-based Zn-air battery (FIG. 5a ) revealed a current densityof ˜160 mA cm⁻² and a peak power density of ˜100 mW cm⁻², comparable tothe Pt/C catalyst (˜110 mA cm⁻² and 75 mW cm⁻²). When normalized to themass of Zn consumed, the specific capacity of Fe-NSCNT-800-based batterywas ˜720 mAh gZn⁻¹ at 10 mA cm⁻², very close to those with 20% Pt/C(˜735 Wh kgZn⁻¹ at 10 mA cm⁻²) (FIG. 6b ). Notably, the potential of theFe-NSCNT-800-based battery (˜1.23 V) is slightly lower than 20% Pt/C(˜1.25 V) at 10 mA cm⁻², but became the same as 20% Pt/C (1.17V) at thehigher current density of 20 mA cm⁻² (FIG. 6c ). Furthermore, nosignificant potential drop was observed for the Fe-NSCNT-800-basedbattery under galvanostatic discharge for 10 h at 10 mA cm⁻² and for 6 hat 20 mA cm⁻² (FIG. 6c ), indicating its good catalytic stability forORR. To meet the specific energy and/or power needs for practicalapplications, multiple Zn-air batteries can be integrated into seriescircuits. As exemplified in FIG. 6d , two Zn-air batteries wereconnected in series to generate a sufficiently high OCP of ˜3V to powera light-emitting diode (LED). The excellent performances of our Fe-NSCNTcatalyst is ascribed to the reinforced synergetic interaction betweenthe enchased Fe₃O₄ nanocrystals and the highly conductive NSCNTbackbone, and its mesoporous morphology which facilitates the O₂ andelectrolyte diffusion to the electroactive sites. From theabove-discussed performance comparison between Fe-NSCNT-800 and 20%Pt/C, it can be clearly seen that Fe-NSCNT-800 holds great potential asan alternative of Pt/C for realizing noble-metal free Zn-air batteries.

FIG. 7 shows that Fe-NSCNT-800 possesses a higher degree of graphiticorder, and thus higher conductivity, than Fe-NSCNT-7, Fe-NSCNT-900 andNSCNT-800, beneficial for ORR.

More specifically it can be seen that the Raman spectrum of all samplesfeature two broad bands. The D band peak centered at 1341 cm⁻¹ isstrongly associated with the defective graphitic band, and the peak at1590 cm⁻¹ (G band) corresponds to the crystalline graphite band. Theintegral intensity ratio (I_(G)/I_(D)) of the G and D bands isconsidered as an gauge for the graphitic ordering degree in carbon. TheI_(G)/I_(D) increases with the increase of pyrolysis temperature andreaches a maximum value of 0.95 at a temperature of 800° C., indicatingthe higher graphitic crystallinity of the carbon structures forFe-NSCNT-800. Different from the common observation in literature thatthe carbon structure becomes more ordered with increased pyrolysistemperature, I_(D)/I_(G) of Fe-NSCNT-900 decreases to 0.86, possiblybecause more heteroatoms are incorporated into the carbon structureswhen annealed at 900° C. lowering the structural orders. Moreover,compared with NSCNT-800 (I_(D)/I_(G)=0.9 and lower 2D peak),Fe-NSCNT-800 possesses a higher degree of graphitic ordering and thushigher conductivity, beneficial for ORR.

FIG. 8 shows that Fe-NSCNT-800 has superior ORR performance with higherlimiting current density than 20% Pt/C. Fe-NSCNT-800 outperformsFe-NSCNT-700 and Fe-NSCNT-900 with enhanced limiting current density.

FIG. 9 proves the Fe-NSCNT-800 ORR catalyst can be used in the acidenvironment.

FIG. 10 shows that Fe-NSCNT-800 has comparable Open Circuit Voltage tocommercial 20% Pt/C.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects as illustrative and notrestrictive.

It will also be appreciated by persons skilled in the art that thepresent invention may also include further additional modifications madeto the method which does not affect the overall functioning of themethod.

Any reference to prior art contained herein is not to be taken as anadmission that the information is common general knowledge, unlessotherwise indicated. It is to be understood that, if any prior artinformation is referred to herein, such reference does not constitute anadmission that the information forms a part of the common generalknowledge in the art, any other country.

The invention claimed is:
 1. A composition of carbon nanotubes,comprising: a plurality of carbonized polymer nanotubes, wherein a wallof the carbon nanotubes is defined with a plurality of nanopores; and aplurality of dopants including iron, nitrogen and sulphur embedding inthe walls of the carbon nanotubes; wherein the iron is in the form ofmagnetite crystals.
 2. The composition according to claim 1, wherein thepolymer is an azo polymer.
 3. The composition according to claim 1,wherein the magnetite crystals anchor to the carbon nanotubes.
 4. Thecomposition according to claim 3, wherein the magnetite crystals areembedded in the wall of the carbon nanotubes.
 5. The compositionaccording to claim 1, wherein the magnetite crystals include iron oxide.6. The composition according to claim 5, wherein the magnetite crystalsinclude Fe₃O₄.
 7. An electrode, filter, absorber, catalyst, and/orsensor comprising the composition of claim
 1. 8. An oxygen reductionreaction catalyst comprising the composition of claim
 1. 9. The dopedcarbon nanotubes of claim 8, wherein the doped carbon nanotubes are usedin an electrode of an electrical energy storage device.
 10. The dopedcarbon nanotubes according to claim 9, wherein the electrical energystorage device further includes a polymer-electrolyte-membrane fuelcell, a metal-air battery or supercapacitor.